In most radio frequency circuit designs, an unbalanced microstrip line architecture is generally adopted and a balun is usually required for converting signals between chips and circuits. However, the use of baluns not only increases costs, but also causes additional signal losses, increased noise, and decreased signal power, which will damage the overall circuit characteristics. Since a radio frequency chip adopts a single-ended input design, therefore the cost can be reduced and the circuit characteristics can be enhanced. For example, a radio frequency receiver has a low-noise single-ended amplifier at its utmost front end, and a next-stage mixer is provided for preventing local oscillation signals leaking to a radio frequency end and an intermediate frequency end or a baseband end. The double balanced architecture is generally used, but the radio frequency input end of the double balanced mixer is a differential input, and thus it is necessary to install a conversion circuit between a low-noise amplifier and a down-conversion mixer for converting a single-ended signal into a differential signal.
In the prior art, the conversion circuit is installed in a chip as shown in FIG. 1, which externally inputs a single-ended signal into a transduction circuit 10, and the current produced by the transduction circuit 10 is passed through a gain control circuit 12 to a load circuit 14 and the input impedance 16 of a next-stage circuit (such as a down-conversion mixer), and the transduction gain of the transduction circuit 10 is gm, wherein the gain of the gain control circuit 12 is a. the load circuit 14 and the next-stage circuit having an input impedance 16 are coupled to a first bonding wire 18 and a second bonding wire 20 respectively. If the impedances of the load circuit 14, the next-stage circuit 16, the first bonding wire 18 and the second bonding wire 20 are ZL, Zm, Z1, and Z2 respectively, the gain of this conventional conversion circuit will be given as follows:
      A    v    =                    V        out                    V        in              =                  a        ·                  gm          ⁡                      (                                                                                Z                    L                                    ⁢                                      Z                    m                                                                                        Z                    L                                    +                                      Z                    m                                    +                                      Z                    1                                    +                                      Z                    2                                                              +                                                                    Z                    1                                    ⁢                                      Z                    m                                                                                        Z                    L                                    +                                      Z                    m                                    +                                      Z                    1                                    +                                      Z                    2                                                                        )                              +                        (                      1            -            a                    )                ·                  gm          ⁡                      (                                                            Z                  1                                ⁢                                  Z                  m                                                                              Z                  m                                +                                  Z                  1                                +                                  Z                  2                                                      )                              
In the abovementioned equation, the term Z1+Z2 will make it difficult to control the gain of the conventional conversion circuit, and the range of the gain of the conventional conversion circuit is limited by two terms
      a    ·          gm      ⁡              (                                            Z              1                        ⁢                          Z              m                                                          Z              L                        +                          Z              m                        +                          Z              1                        +                          Z              2                                      )              ⁢          ⁢  and  ⁢          ⁢            (              1        -        a            )        ·                  gm        ⁡                  (                                                    Z                1                            ⁢                              Z                m                                                                    Z                m                            +                              Z                1                            +                              Z                2                                              )                    .      Further, the impedance of the conventional conversion circuit as shown in FIG. 1 is equal to the sum of the load circuit 14 and the first bonding wire 18, and the gain of the conventional conversion circuit cannot be predicted. In addition, it is necessary to have a very good AC ground for Point P at the junction of the load circuit 14 and the first bonding wire 18, or else the control of gain will not be accurate, and the output voltage Vout is closely related to the wire bonding, manufacturing process, and packaging, and thus will be varied easily by these factors.